The present invention is directed toward an electroslag pre-melting furnace and more particularly toward a pre-melting furnace which includes means for controlling the atmosphere therein.
The electroslag melting process was first invented, developed and put into full production by R. K. Hopkins in the United States during the period between 1930 and 1960. This process employs a consumable electrode which is immersed in a pool of molten slag supported at the top of the resultant solidifying ingot enclosed within a cold-walled mold or crucible.
Alternating (or sometimes direct) current flows down the consumable electrode through the slag, down the ingot and back to the power supply. Preferably, the current flows back to the power supply in a coaxial manner to the top of the crucible such as shown in co-pending application Ser. No. 616,365 filed Sept. 24, 1975, now U.S. Pat. No. 4,032,705. This current, normally in the range of 1,000 amps per inch of ingot diameter, drops from fifteen to forty volts across the slag (or flux) pool thereby producing hundreds of kilowatts of melting power which consumes the tip of the electrode.
As a result of the foregoing, molten metal droplets form on the immersed electrode tip, detach themselves and fall through the molten flux pool to the ingot which is forming there below. As the metal droplets pass through the flux pool, they undergo chemical refinement. Progressive solidification of the ingot formed by this method leads to the physical isotropy and high yield associated with all consumable electrode processes.
As is known in the art, most electroslag ingots of 24 inch diameter and larger are started by pre-melting a slag of suitable chemistry and pouring a six to eight inch deep pool of this molten slag into the bottom of the crucible. The electrode tip is then immersed to a depth of a half an inch or so into this molten pool. The melting current flowing through the molten flux raises its temperature until the electrode begins to melt.
Molten flux (or slag) starting, as this technique is known, gives much higher utilization of the consumable (electroslag) furnace and better ingot yield than "dry" or cold starting because ingot bottom losses are minimized.
In the past, flux pre-melting furnaces have always been open to the atmosphere. These have consisted primarily of air induction furnaces with the graphite crucible acting as a susceptor. More recently, A. C. resistance furnaces have been employed. These include one or more graphite electrodes in a single phase system, or three electrodes in a three phase system for larger units, which electrodes function as submerged melters in a graphite monolithic crucible or graphic brick lining.
These prior art systems, however, have several serious drawbacks. For example, at elevated temperatures (most electroslag fluxes have melting points in the 2,500.degree. to 3,500.degree. F. range) and in the presence of air (or oxygen) graphite erodes quite rapidly which leads to low heat life of the crucibles (20 to 30 heats) and frequent electrode replacement.
Even worse, a substantial part of the eroded graphite, from both the lining and the electrodes, dissolves in the molten flux. It is then poured into the electroslag crucible and is transferred to the bottom few inches of the ingot being built up. In the case of a low carbon alloy steel heat, carbon pick-up from this source can easily scrap all or part of the ingot.